Along with an advance in commercial application of liquid crystal displays, liquid crystal displays of an active matrix type capable of displaying images of excellent quality have come by now to occupy a position in the mainstream of the market.
The active matrix type liquid crystal display described above comprises thin-film transistors (TFTs), diodes, or nonlinear resistance elements of a metal-insulator-metal (referred to hereinafter as "MIM") structure composed of three layers consisting of metal-insulator-metal or metal-insulator-transparent and electrical conductor, as switching elements for each of liquid crystal display electrodes for displaying images.
The MIM elements described above is generally composed of a Ta-Ta.sub.2 O.sub.5 -Cr or Ta-Ta.sub.2 O.sub.5 -ITO structure. Herein, Ta refers to a tantalum film, Ta.sub.2 O.sub.5 a tantalum oxide film, Cr a chromium film, and ITO an indium tin oxide film.
With a liquid crystal display using MIM elements, images are displayed by switching on and off a liquid crystal layer connected in series with the MIM elements by taking advantage of a nonlinear voltage-current characteristic of the MIM elements.
Now referring to FIGS. 29 to 32, the structure of a conventional liquid crystal display panel having nonlinear resistance elements composed of the Ta-Ta.sub.2 O.sub.5 -ITO structure is described hereafter.
As shown clearly in FIG. 32, the MIM element comprises a tantalum (Ta) film as a lower electrode 103 formed on a first substrate 102, a tantalum oxide (Ta.sub.2 O.sub.5) film as an insulation film 104 formed on the lower electrode, and a transparent and electrically conductive film composed of an indium tin oxide (ITO) film as an upper electrode 105 formed on the insulation film, all these films together constituting a nonlinear resistance element.
In addition, the MIM element is provided with a display electrode 106 composed of an indium tin oxide film. Data signals dependent on the contents of display are applied on the display electrode 106 via the nonlinear resistance element by a signal electrode 107 composed of a tantalum film and a tantalum oxide film.
This liquid crystal display is provided with the first substrate 102 on which the nonlinear resistance elements are formed and a second substrate 109 (refer to FIG. 29) having opposite electrodes 110 (as indicated by phantom lines in FIG. 30) formed in such a way as to face the display electrodes 106 formed on the first substrate 102.
After applying liquid crystal-molecular alignment treatment to the surfaces of the first substrate 102 and the second substrate 109, the two substrates are bonded together with a sealing portion 108 such that the surfaces of the both substrates face each other at a predetermined spacing, and liquid crystals are sealed in a gap formed therebetween, thus forming a liquid crystal display. A region surrounded by a phantom line 118 as indicated in FIG. 29 and a solid line 118 as indicated in FIG. 30 represents a display region of the liquid crystal display.
However, the liquid crystal display having the conventional nonlinear resistance elements described above poses a problem of an after-image phenomenon occurring when an image displayed is changed in the course of driving the liquid crystal display.
Referring to FIG. 33, the after-image phenomenon is described. Herein, the liquid crystal display is assumed to display images in "normally white" mode.
FIG. 33 indicates variation in transmissivity of light when an applied voltage for a random pixel is varied for every 5 minutes. Specifically, a voltage (V1) for providing a display of 50% transmissivity is applied for first 5 minutes (unselect period: T1), then a voltage (V2) for providing a display of 10% transmissivity is applied for another 5 minutes (select period: T2), and further a voltage (V3) at the same level as that of the voltage (V1) applied for the first unselect period T1is applied for yet another 5 minutes (unselect period: T3).
The after-image phenomenon is a phenomenon wherein a difference (.DELTA.T) in transmissivity between the unselect period T1 and the unselect period T3 develops although the voltages applied for respective periods are equal. With the liquid crystal display described above, the difference .DELTA.T in transmissivity was found to be 5%.
The occurrence of the after-image phenomenon results in the display of an image with its contents different from those of an originally intended image.
Therefore, an image sticking phenomenon, that is, the after-image phenomenon degrades considerably the quality of images displayed by the liquid crystal display, posing a serious problem in commercial application thereof.
A primary cause for the occurrence of the after-image phenomenon is a d-c voltage component of a voltage applied on the liquid crystal layer when driving the liquid crystal display. Owing to the d-c voltage component, a polarization phenomenon of alignment layers used for aligning liquid crystal molecules in a predetermined direction and the degradation of liquid crystals themselves occurs, resulting in the occurrence of the after-image phenomenon.
FIG. 34 is a graph showing a current-voltage characteristic (I-V characteristic) of a non-linear resistance element composed of a "tantalum film-tantalum oxide film-indium tin oxide film" structure according to a conventional structure.
As shown in the figure, variation in current value differs considerably depending on the polarity of an applied voltage, demonstrating an asymmetrical current-voltage characteristic with respect to a voltage at zero.
As a means for achieving an improvement on the asymmetrical current-voltage characteristic, it is conceivable to replace the indium tin oxide film composing the upper electrode 105 of nonlinear resistance elements with such a metal film as a chromium (Cr) film, a titanium (Ti) film or the like.
Such replacement of the indium tin oxide film with the chromium film or the titanium film in forming the upper electrode 105 can moderate to some extent the asymmetry of the current-voltage characteristic as shown in FIG. 34, but is still far from achieving a fully symmetrical current-voltage characteristic.
Further, an offset driving method is proposed to prevent the d-c voltage component from being applied on the liquid crystal layer through the nonlinear resistance elements having the asymmetrical current-voltage characteristic. The offset driving method is described hereafter with reference to FIG. 35.
As shown in FIG. 35, the offset driving method is a method of driving the liquid crystal display by varying voltages applied in a select period (Ts) and a hold period (Th), respectively, depending on the polarity of an electric field, that is, a (+) field or a (-) field so that the d-c voltage component will not be applied on the liquid crystal layer by compensating for the asymmetric characteristic of the element with a varying driving voltage.
Voltages applied in the select period (Ts) are denoted Va1 and Va2, and voltages applied in the hold period (Th) are Vb1 and Vb2.
With the offset driving method as shown in FIG. 35, the d-c voltage component of a voltage applied between the display electrodes 106 and the opposite electrodes 110, disposed facing each other, with the liquid crystal layer sandwiched therebetween can be reduced.
However, asymmetrical voltages, for example, Vb2 and Vb1 are applied on the signal electrodes 107 as shown in FIGS. 30 and 31, but symmetrical voltages are applied on the liquid crystal layer. Consequently, a voltage between the signal electrodes 107 on the first substrate 102 composing the MIM elements and the display electrodes 106 contains the d-c voltage component. Furthermore, the d-c voltage component occurs similarly between the opposite electrodes 110 and the signal electrodes 107.
As a result, with nonlinear resistance elements having the asymmetric current-voltage characteristic, it was impossible to reduce sufficiently the d-c voltage component impressed on the liquid crystal layer, eliminating the after-image phenomenon completely.
Therefore, it is an object of the present invention to provide an liquid crystal display capable of displaying images of excellent quality without the effect of the after-image phenomenon by reducing a d-c voltage component impressed on the liquid crystal layer in its nonlinear resistance elements having an asymmetrical current-voltage characteristic.